chore(ai): add prepare workspace skill

.claude/skills/prepare-workspace-for-plan/SKILL.md

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+---
+name: prepare-workspace-for-plan
+description: Use when you have a plan file ready and need to create an isolated git worktree for implementation - creates worktree in parent directory following project conventions and moves the plan file
+---
+
+# Prepare Workspace for Plan
+
+Use this skill when you have created or refined a plan and need to set up an isolated workspace for implementation.
+
+## When to Use
+
+- After creating/finalizing a plan in the `plans/` directory
+- Before starting implementation of a multi-phase plan
+- When you need an isolated branch for a feature or fix
+
+## Prerequisites
+
+- A plan file exists in the current workspace's `plans/` directory
+- You are in a git repository that supports worktrees
+- The parent directory is the standard location for worktrees (e.g., `/path/to/vikunja/`)
+
+## Steps
+
+### 1. Determine Workspace Name
+
+Choose a name following the project convention:
+- `fix-<description>` for bug fixes
+- `feat-<description>` for new features
+
+The name should be kebab-case and descriptive but concise.
+
+### 2. Create the Git Worktree
+
+```bash
+# From the current workspace (e.g., main/)
+git worktree add ../<workspace-name> -b <branch-name>
+```
+
+The branch name should match the workspace name.
+
+### 3. Create Plans Directory and Move Plan
+
+```bash
+mkdir -p ../<workspace-name>/plans
+mv plans/<plan-file>.md ../<workspace-name>/plans/
+```
+
+### 4. Verify Structure
+
+```bash
+ls -la ../<workspace-name>/plans/
+```
+
+## Example
+
+```bash
+# Create worktree for position healing fix
+git worktree add ../fix-position-healing -b fix-position-healing
+
+# Move the plan
+mkdir -p ../fix-position-healing/plans
+mv plans/positioning-fixes-detection.md ../fix-position-healing/plans/
+```
+
+## Result
+
+After completion, you'll have:
+```
+parent-directory/
+├── main/                    # Original workspace
+├── <new-workspace>/         # New worktree
+│   └── plans/
+│       └── <plan-file>.md   # Your plan
+└── ...                      # Other existing worktrees
+```
+
+## Notes
+
+- The new worktree shares git history with main but has its own working directory
+- Changes in the new worktree won't affect main until merged
+- Plans are not committed to git (see `.gitignore`)
+- Remember to switch to the new workspace directory to begin implementation